net: ethtool: Add attributes for cable test TDR data

Some Ethernet PHYs can return the raw time domain reflectromatry data.
Add the attributes to allow this data to be requested and returned via
netlink ethtool.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>

v2:
m -> cm
Report what the PHY actually used for start/stop/step.

Signed-off-by: David S. Miller <davem@davemloft.net>